Fastening of sheet material

ABSTRACT

A fastener such as a self piercing rivet ( 1 ) is inserted into sheet material ( 19 ) without full penetration such that the deformed end of the rivet remains encapsulated by an upset annulus of the sheet material ( 19 ). The sheet material ( 19 ) is clamped between a nose ( 17 ) and a die ( 16 ) of a riveting machine. Once the sheet material ( 19 ) is clamped, the distance between the nose ( 17 ) and the die ( 16 ) is controlled during the riveting operation by a restraint device ( 20, 23 ). This technique ensures that reaction forces generated in the joint between the nose ( 17 ) and the die ( 16 ) during riveting are absorbed in the joint as compressive stresses during the riveting operation so that the resulting joint has improved fatigue life. Furthermore, the rivet insertion load imparted into the C frame as a reaction force is applied back into the joint as a post rivet-insertion clamping load. Alternatively, a separate clamping force is applied after the rivet ( 1 ) has been inserted.

The present invention relates to a method and apparatus for fasteningsheet material by self-piercing riveting or clinching. The term“clinching” is also known as “press joining” or “integral fastening”.

Methods and apparatus for riveting of the kind in which a self-piercingrivet is inserted into sheet material without full penetration, suchthat the deformed end of the rivet remains encapsulated by an upsetannulus of the sheet material are known.

FIG. 1 is a diagrammatic section of an example of a riveted joint madeby such a riveting method in accordance with the invention. A rivet 1has a head 2 and a shank 3 terminating in an annular edge 4. The shank 3is initially cylindrical but is flared outwardly into the illustratedshape as the rivet is driven into two overlapping sheets 5,6 located ona suitably shaped die. As shown, the shank and the edge of the rivet 1remain embedded in the sheet material 5,6 after the rivet has been set.

An improved self-piercing riveting method is described in our EuropeanPatent No. 0675774. In this method the sheet material is clamped withsubstantial force during the riveting operation between a nose of theriveting machine and the die in the region around the rivet insertionlocation so that there is minimal distortion of the sheet materialduring the riveting operation. This method has been proved to increasethe strength of the riveted joint and reduce the depth of the annularvalley 7. However, the relatively high level of clamping force requiredto achieve the improved joint characteristics means that a significantpressure of hydraulic fluid or a heavy-duty spring is required to applythe force. Furthermore, if reaction forces within the joint resultingfrom rivet insertion exceed the clamping force, the nose will be pushedback up away from the die. This results in a reduction of the potentialresidual compressive stress that could be imparted to the region aroundthe rivet.

Joining two sheets of metal by clinching is known, whereby two sheets ofmetal are deformed into locking engagement using a punch-and-diecombination.

An improved clinching method is described in our European Patent No.0614405. In this method a hollow rivet or tubular slug is inserted intoa clinched joint between sheets and the inner end of a shank of therivet is outwardly deformed within the clinched joint in such a way thatit does not penetrate the panels.

In both the above-described methods a C-frame is used to support theriveting apparatus and die. A lower limb of the C-frame supports the dieand, in use, deflects a certain distance during the riveting operationas a result of the rivet insertion and clamping forces. This means thatin hydraulic clamping systems top-up hydraulic fluid is generallyrequired to maintain the required level of clamping. The slow responseof hydraulic fluid systems to the demand for extra loading leads torelatively long cycle times.

It is an object of the present invention to obviate or mitigate theaforesaid disadvantages and to provide for an improved method andapparatus for fastening sheet material by self-piercing riveting orclinching.

According to a first aspect of the present invention there is provided amethod for inserting a fastener into sheet material comprising insertingthe fastener into at least one sheet without full penetration such thata deformed end of the fastener remains encapsulated by an upset annulusof the sheet material, the sheet material being disposed between a noseand a die of fastening apparatus and the fastener being inserted intothe sheet material by means of a punch that is reciprocal relative tothe nose, characterised in that, after the fastener is inserted thepunch is retracted and a clamping force is applied to the sheet materialbetween the nose and die in the region around the fastener insertionlocation so as to reduce deformation of the sheet out of its plane inthe region around the fastener insertion location.

The clamping of the sheet after insertion of the fastener in this wayensures that favourable compressive stresses are built into the regionaround the fastener insertion location and, in the case where one ormore sheets are being joined, also ensures that fatigue performance ofthe joints significantly improves in, comparison to joints produced byconventional fastening methods. Retraction of the punch ensures thatclamping of the sheet after rivet insertion is applied only by the nose.

The nose may be supported by a support member, movement of the noserelative to the support member in a direction away from the die beingprevented during the fastening operation.

The position of the nose in relation to the die is controlled such thatvery little or no local thickening or swelling of the sheets in theregion around the rivet insertion location is permitted. The methodmeans that the sheet material displaced during the rivet insertionoperation is constrained within a substantially fixed distance betweenthe nose and the die. This constraint results in increased residualcompressive stress being induced into the joint area as a result of therivet insertion, thereby providing improved fatigue life of the rivetedjoint.

The distance between the nose and the die is preferably controlled byrestraining the nose of the riveting machine so that it is preventedfrom being pushed back away from the die during the riveting operation,regardless of the force applied. However in the event of deflection of asupport frame (such as a C-frame) owing to rivet setting loads, the noseis still able to move so as to follow the die.

A clamping force may also be applied to the sheet material prior to orduring insertion of the fastener.

Preferably the clamping force and the force required to insert thefastener are derived from the same actuator.

The clamping force applied after insertion of the fastener may be variedby varying the force applied to insert the fastener.

According to a second aspect of the present invention there is provideda apparatus for by inserting a fastener into sheet material without fullpenetration such that a deformed end of the fastener remainsencapsulated by an upset annulus of the sheet material, said apparatuscomprising a nose in which is disposed a reciprocal punch, means forfeeding fasteners successively to the nose for insertion by the punchinto the sheet material, a die aligned with the punch for deforming thefastener inserted, the sheet material being disposed between the noseand die during the fastening operation, the nose being supported by asupport member, characterised in that there is provided means forretracting the punch into the nose after insertion of the fastener sothat the punch does not project therefrom and means for applying aclamping force to the sheet material after insertion of the fastener,the force being applied between the nose and die in the region aroundthe fastener insertion location so as to reduce deformation of thesheets out of their planes in the region around the fastener insertionlocation.

In one embodiment, when the punch has travelled its full extent and therivet is inserted, a substantial reaction force will have beenestablished in a supporting frame such as, for example, a C-frame. Thisforce is an equal and opposite reaction to the force applied by thepunch. With the nose position controlled, retracting the punch away,from the formed joint will result in the frame reaction load beingtransferred from the punch to the nose. This transfer of load results inthe sheet material surrounding the rivet being squeezed between the noseand die with the same substantial force initially used to insert therivet. Such post rivet-insertion clamping provides for increasedresidual compressive stresses being imparted into the joint utilisingforces already available as a result of rivet insertion and henceremoves the need for separate or additional loading. Further, postrivet-insertion clamping allows for the use of clamping forces rangingfrom zero to moderate during rivet insertion yet still achieves all ofthe advantages of a joint clamped with substantial force prior to rivetinsertion and has been shown to exceed the fatigue results produced byusing moderate or high clamping forces.

The restraint device is preferably connected to the nose so as toprevent movement of the nose away from the die during the rivetingoperation. It may take the form of a mechanical linkage connectedbetween the nose and a fixed surface, the linkage being acted upon by anactuator so that its movement, and therefore movement of the nose awayfrom the die is prevented during the riveting operation. However thedevice still allows the nose to follow die deflection.

Alternatively the restraint device may be a rotary nut threadedlyengaged with a threaded surface of the nose, the nut being rotatable toenable movement of the nose towards and away from the die, rotation inone direction being prevented during the riveting operation so as toprevent movement of the nose away from the die. However, rotation in theother direction, allowing the nose to follow the die, is permitted.

In an alternative preferred embodiment the restraint device is a wedgemember disposed between the nose and supporting frame, the wedge memberbeing moveable between an operative position in which it preventsmovement of the nose away from the die operation (but still allowing thenose to follow the die if required) and a released position in whichsuch movement is permitted enabling the mechanism to be retracted at theconclusion of the cycle.

In a further alternative preferred embodiment the restraint device isprovided by a supply of hydraulic fluid through an inlet connector tothe riveting device, the hydraulic fluid applying pressure to controlthe movement of the nose towards the die, the inlet connector beingfitted with a check valve or similar type device that prevents releaseof fluid pressure and therefore movement of the nose away from the dieduring the riveting operation.

According to a third aspect of the present invention there is provided apanel clinching method wherein two or more sheets of material aredeformed into locking engagement, the sheet material being disposedbetween a nose and a die of fastening apparatus, the sheet materialbeing deformed by means of a punch that is reciprocal relative to thenose, characterised in that, after the sheets are deformed by the punch,the punch is retracted and a clamping force is applied to the sheetmaterial between the nose and die in the region around the punchinsertion location so as to reduce deformation of the sheet out of itsplane in the region around the punch in-sertion location.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a section of a riveted joint made by the fastening method ofthe present invention;

FIGS. 2 and 3 are diagrammatic illustrations of a first embodiment of ariveting machine of the invention shown mounted on a C-frame;

FIG. 4 is a diagram showing the internal stresses of a joint beingriveted in accordance with the method of the present invention;

FIG. 5 is a diagrammatic side view of a second embodiment of a rivetingmachine of the present invention;

FIG. 6 is a diagrammatic side view of a third embodiment of a rivetingmachine of the present invention;

FIG. 7 is a diagrammatic representation of a fourth embodiment of ariveting machine of the present invention;

FIGS. 8a to 8 h are longitudinal sectioned views of a rivet setter anddie of the present invention showing in a sequence of chronologicalsteps a method of fastener insertion in accordance with the presentinvention;

FIG. 8i is an exploded view of a plunger and punch assembly of the rivetsetter of FIGS. 8a to 8 h and 9 a to 9 e; and

FIGS. 9a to 9 e are longitudinal sectioned views of a rivet setter anddie showing a chronological sequence of an alternative method offastener insertion in accordance with the present invention.

Referring now to the drawings, the riveted joint of FIG. 1 has alreadybeen described as an example of the kind of joint that is produced bythe fastening method of the invention.

FIGS. 2 and 3 show a riveting machine 10 mounted on a conventionalC-frame 11. An upper limb 12 of the C-frame 11 supports a fixed maincylinder 13 in which is received a retractable clamping cylinder 14 ofthe machine. The main cylinder 13 provides hydraulic pressure foractuation of the clamping cylinder and a rivet punch (not shown) that iscoaxially slidable therein. A lower limb 15 of the C-frame 11 supports adie 16 directly below the clamping cylinder 14. The clamping cylinder 14terminates in a nose 17 the end surface of which defines an annularclamping surface 18 that urges two overlapping sheets 19 against the die16.

The movement of both the clamping cylinder 14 and the punch is driven bya hydraulic fluid pressure as is well known and discussed in ouraforementioned European Patent. However, other punch drive mechanismsmay be used such as an electrically powered screw assembly. Furthermore,the clamping cylinder may alternatively be driven by an electricallypowered actuator or via a spring from the punch.

A mechanical linkage 20 is connected to the C-frame 11 and to the nose17 in order to maintain the extension of the nose 17 towards the die 16during the riveting operation. The linkage 20 comprises a first linkmember 21 fixed at one end to the upper limb 12 of the C-frame 11 and asecond link member 22 connected to the nose 17, the two link members 21,22 being pivotally interconnected at their other ends. The linkage 20 iscontrolled by an actuator 23 connected between the C-frame 11 at alocation intermediate the upper and lower limbs 12, 15 and the pivotbetween the first and second link members 21, 22. In use, the linkage 20moves in synchronism with the descent of the clamping cylinder 14, or,alternatively, is acted upon by the actuator 23 to advance the clampingcylinder 14, between an extended position, shown in FIG. 3, in which thenose 17 is in clamping contact with the sheets 19 and a contractedposition, shown in FIG. 2, in which the nose 17 is retracted towards theupper limb 12 of the C-frame 11. The actuator 23 serves to drive andhold the mechanical linkage 20 in the extended position so thatretraction of the nose 17 relative to the C-frame 11 is prevented untilsuch time as the riveting process is complete and the clamping pressureis released whereupon the actuator 23 is released.

The riveting operation is performed as follows. A rivet of the kindshown in FIG. 1 is delivered to the end of the nose 17 in a conventionalmanner ready for insertion into the sheets 19. The clamping cylinder 14descends downwards from the retracted position shown in FIG. 2 to theposition shown in FIG. 3 and applies a light to moderate clamping forcebetween the nose 17 and the die 16. The mechanical linkage 20 eitherextends simultaneously with, or drives the descent of the clampingcylinder 14 and a restraining force is applied by the actuator 23 to thelinkage 20 so as to prevent reverse movement of the nose 17 during theriveting operation, thereby controlling the position of the nose 17 inrelationship to the die 16. As soon as the pre-selected clamping forceis reached a pressure switch or load cell etc. (not shown) signals thepunch to advance for the riveting operation. The clamping force may bemaintained throughout the rivet insertion or may be reduced or increasedby means of varying the pressure in the actuator. The punch thendescends within the clamping cylinder 14 to insert the rivet into thesheets 19.

The timing and co-ordination of above operation is conducted under thecontrol of a programmable logic controller or a similar automaticcontrol device. It will be understood that the riveting operation may besupplemented by the inclusion of a coining ring and/or an adhesiveapplied to the nose as described in our aforementioned European Patent.

The insertion of the rivet is illustrated in FIG. 4. Whilst the clampingforce is applied by the nose 17 to the sheets 19 there is acorresponding equal and opposite reaction force applied by the die 16 tothe sheets 19. In addition to this there are -further reaction forcesgenerated by the tendency of the sheets to deform as the rivetpenetrates the sheets under action of the punch. Initially sheetmaterial is drawn into the die cavity by the rivet as it is inserted. Asthe rivet is fully inserted and fills the die cavity sheet material isdisplaced out of the cavity. This displaced material generates reactionforces against the nose and the die. The reaction forces, illustrated bythe arrows F in FIG. 4, exceed the clamping force and act against thenose 17 which would normally be deflected outwardly away from the die 16by the reaction forces generated by the deformation of the sheets 19.However, since the nose 17 is restrained from moving away from the die16 by operation of the linkage 20 when in the clamping position, it isnot deflected and the joint is correspondingly compressed. Thecompressive stress applied during the riveting process provides thejoint with improved fatigue life.

The advance of the punch during insertion of the rivet into the jointapplies increased force to the sheets and the die 16 and lower limb 15of the C-frame 11 tend to deflect slightly downwardly under theincreased load. The deflection of the C-frame is followed by the nose 17in order that the clamping force is maintained. The mechanical linkage20 is able to extend slightly further to allow this additional travel ofthe nose 17. When the riveting force is removed, (i.e. the insertion ofthe rivet is complete and the punch is retracted into the nose) butwhilst the clamping force is still maintained, the lower limb 15 of theC-frame 11 will attempt to spring back to an equilibrium positionagainst the nose 17. In the absence of the linkage 20 the nose 17 wouldbe deflected upwards until the force (applied by the clamping pressure)is equal to the reaction force of the lower limb of the C-frame.However, in the presence of the linkage 20 the nose 17 is not moveablerelative to the C-frame and all of the reaction force from the rivetingoperation is thus transmitted via the die 16 back into the joint whichbecomes squeezed between the die 16 and nose 17 by a force which issubstantial and approximately equal to the rivet insertion force(typically 2-5 tonnes less the initial clamping force). Since the punchis retracted fully into the nose the squeezing force is applied only bythe nose. This results in both flattening of the joint and imparting offavourable compressive stresses providing advantageous strength andfatigue performance. After this post rivet-insertion ‘squeeze’ of thejoint has been allowed to occur fully, the nose restraining device isdisengaged so that it can retract ready for the next cycle.

The riveting method of the present invention allows for improved jointcharacteristics such as improved fatigue life without the need for aseparately applied high clamping force. Research has established thatinitial clamping forces of zero or low to moderate such as 100 lbf or soare sufficient to ensure effective riveting using the method of thepresent invention although in practice higher or lower forces may beused. Moreover, additional top-up hydraulic fluid is not required tocompensate for deflection of the C-frame.

Alternative designs of riveting machines are shown in FIGS. 5, 6 and 7.Components identical to those of the previous figure are given the samereference numerals. In the example shown in FIG. 5 the mechanicallinkage is replaced by a spring-loaded wedge-shaped collar 30 that ismounted between the upper limb 12 of the C-frame 11 and the clampingcylinder 14. The collar 30 permits descent of the clamping cylinder 14but prevents any reverse motion by a jamming effect until rivetingprocess is complete. When the rivet has been inserted into the sheets 19and the clamping force is released the wedge-shaped collar 30 is movedout of engagement with the C-frame 11 so as to permit ascent of theclamping cylinder 14 and therefore movement of the nose 17 away from theriveted joint.

In the embodiment of FIG. 6 the main cylinder has an exterior screwthread formation 40 that engages with a complementary internal thread ofa rotary nut 41 disposed immediately below the upper limb 12 of theC-frame 11. The nut 41 is drivingly connected via a belt 42 to a motor43 that is supported on the side of the upper limb 12. In operation, thetravel of the clamping cylinder 14 relative to the C-frame 11 iseffected by rotation of the nut 41 by the motor 43. The nut 41 isrotated until the nose 17 is in a clamping position on the sheets. Thethreads are designed to be of shallow pitch so that they tend to lock toprevent reverse movement of the main cylinder 14 during the rivetingoperation. Alternatively, the clamping cylinder may be hydraulically orotherwise driven and the screw thread acts as a complimentary lock toprevent reverse movement.

The riveting machine of FIG. 7 is substantially similar to thatdescribed in our aforementioned European Patent No. 0675774. A punch 50is carried by a plunger 51 and is slidable within the main cylinder 14under the influence of hydraulic pressure admitted through an inletconnector 52 of the main cylinder. The lower part of the cylinder 14houses a slidable clamping cylinder 53 that terminates in a nose 54 theend face of which provides a clamping surface 55. The clamping pressureis provided by hydraulic fluid admitted through an inlet connector 56 inthe main cylinder 14. In order to retract the clamping cylinder 14 andrelease the clamping pressure, a check valve 57 located in the inletconnector 56 is opened and fluid is allowed to escape back through inletconnector 56. The machine differs from conventional designs in that thehydraulic check valve 57 (or an equivalent device) is located in orclose to the inlet connector 56 so as to limit the volume of hydraulicfluid that is held at the clamping pressure. This serves to restrainupward movement of the nose 54 and so enables the distance between thenose 54 and the die to be controlled.

It is to be understood that all the above designs can prevent the lowerlimb of the C-frame springing back to its equilibrium position after theriveting force is removed as described above in relation to theembodiments shown in FIGS. 2 and 3.

In a further alternative embodiment (not shown) a single actuator (forexample a hydraulic cylinder or an electric motor) drives both the punchand nose. Once the nose contacts the sheets to be joined a relativelylight spring force (e.g. 20 lbf) is applied whilst the punch continuesto advance within the nose. The punch comes into contact with a rivetthat has been delivered to the nose and drives it into the sheets. Atthe moment the rivet comes into contact with the upper sheet there isstill only a relatively low clamping force being applied to preventrattling of the riveting machine relative to the sheets. This very lowforce allows the sheet material being joined to flow so that it isdragged towards the die and into the die cavity by the advancing rivetresulting in the material immediately around the rivet being deformedinto an annular valley. This occurrence allows more sheet material toflow into the die and results in relatively low tensile stresses beingset up in the sheet material in the region around the rivet insertionlocation. As the head of the rivet is pressed flush with the uppersurface of the top sheet a shoulder or stop on the punch abuts acomplementary shoulder on the nose and prevents further travel of thepunch relative to the nose. Thus any additional force applied by theactuator to the punch is shared between the punch and nose and serves toincrease the clamping force between the nose and punch combination andthe die. Both the rivet insertion force and the clamping force cause thelower limb of the C-frame to deflect as discussed above and thisdeflection is maintained as long as the punch remains in the extendedposition.

At a predetermined time after the rivet insertion stroke of the punch iscomplete, the punch is retracted whilst the nose is retained in positionusing any appropriate restraining mechanism such as any one of thosedescribed above. The force applied by the actuator is now applied to thejoint solely by the end surface area of the nose and C-frame reactionpushes the die and sheets towards the fixed nose so as to clamp or“squeeze” the sheets in the region around the rivet insertion location.Since the punch is retracted no load is imparted to the rivet at thispoint. This squeezing serves to flatten out the distortion of the sheetsout of their planes that occurred during rivet insertion in the regionaround rivet insertion. In addition, it imparts compressive stressesinto the sheet material around the rivet shank. Once the squeezing forcehas stabilised the nose is then retracted as before and the punch isreset for the next stroke.

The amount of clamping or squeezing force can be varied in several ways.The clamping force may be increased in magnitude by increasing theactuator load prior to lifting the punch once the rivet has beeninserted. This does not drive the rivet further into the sheet materialas advance of the punch relative to the nose is prevented by theabutting shoulders. The additional load is thus imparted to the C-framewhich deflects further. Once the punch is retracted the C-frame reactsand the load is transferred into clamping of the joint. Similarly, theclamping force may be reduced by reducing the actuator load prior toretraction of the punch so that a lower force is reacted by the C-frameand imparted into the joint. The clamping force can be measured by meansof a load cell associated with the actuator so that the load applied bythe actuator can be controlled to achieve the required clamping force.This arrangement is advantageous as no additional stroke or loading ofthe actuator is required to impart the clamping force, thereby saving oncycle time and power.

The duration of the clamping force imparted into the joint is dependenton the stiffness of the C-frame and the control device (referred toabove) is designed to take such factors into account during apreliminary calibration stage that establishes the required actuatorload for a desired clamping force and a given C-frame stiffness.

By reducing the initial clamping force (i.e. that prior to rivetinsertion) in favour of a post rivet-insertion force the impact on thenose of the riveting apparatus is reduced. In the embodiment describedabove the clamping force may be increased gradually in stages duringand/or after rivet insertion thereby reducing the risk of impact damageto the nose. Furthermore, as the clamping force and the rivet insertionforce are applied via the same actuator only a single supply ofhydraulic fluid is required. This eliminates the need for a two-stageoperation of clamping then riveting as in existing riveting apparatus.Such a single stage operation is ideally suited to electric actuatorsthat hitherto have proven unsuitable for riveting operations in that thejoints produced have been of poor quality.

The rivet setter and die shown in FIGS. 8a to 8 i has a single actuatorand can be used in the embodiment described immediately above or may beused in such a way that the clamping force is imparted to the sheetmaterial joint after rivet insertion in a different manner as will nowbe described. The operation of the apparatus is shown as a series ofchronological steps.

A cylindrical support tube 70 carries a clamping cylinder 71 that iscoaxially slidable therein. The clamping cylinder 71 in turn supportsinternally a coaxial plunger 72 that is slidable therein and carries apunch 73. The lower part of the clamping cylinder 71 terminates in anexternal nose 74 the end face 75 of which provides a clamping surfacefor the sheet material 5. The upper end of the clamping cylinder 71defines an annular abutment shoulder 77 the purpose of which will bedescribed below. The nose 74 is internally configured to define a guidebush 78 of relatively narrow diameter that receives the punch 73 incoaxial alignment and guides the punch 73 during relative slidingmovement thereof against the bias of a coaxial punch spring 79. In therelaxed condition the punch spring 79 urges the punch 73 away from theoutlet of the nose 74.

The plunger 72 is moveable in the clamping cylinder 71 by means of theshaft 80 of a linear actuator (not shown) the end of which supports anannular shoulder 81 for abutment with the shoulder 77 on the clampingcylinder 71. The respective shoulders 77, 81 serve to limit the extentof travel of the actuator shaft 80 in the support tube 70. Inside theclamping cylinder 71 the plunger 72 has a radially outwardly definedstep that serves as a stop 82 that, in use, co-operates with an abutmentshoulder 83 at the top of the guide bush 78 so as to limit the extent oftravel of the plunger 72. A compression spring 84 is coaxially disposedbetween the actuator shaft 80 and the top of the clamping cylinder 71.

Interposed between the plunger 72 and the punch 73 is a rotary cam 85that is actuable to allow the punch 73 to retract slightly into the nose74 under the influence of the punch spring 79 as will be describedbelow. The cam 85 is mounted on a pin 86 that rides in a slot (notshown) during reciprocal motion of the plunger 72 and punch 73. When itis desired to retract the punch 73 the cam 85 is rotated about thelongitudinal axis of the rivet setter so as to reduce the distancebetween the proximate ends of the plunger 72 and punch 73. The slot isconfigured to allow such rotation to occur at a predetermined axialdistance along the rivet setter.

At rest the actuator shaft 80, plunger 72 and punch 73 are retracted inthe rivet setter as shown in FIG. 8a In order to effect rivet insertion,the actuator shaft 80 is first advanced into the support tube 70 therebyforcing the clamping cylinder 71 to extend until the nose 74 is incontact with the sheet materials as shown in FIG. 8b. The nose 74 is nowprevented from further movement and continued advance of the actuatorshaft 80 compresses the compression spring 84 between the actuator shaft80 and the plunger 72. The only clamping force applied to the sheetmaterial around the rivet insertion location at this stage is thatapplied by the compression spring 84. The advance of the actuator shaftcauses movement of the plunger 72 within the clamping cylinder 71 whichin turn urges the punch 73 to move against the biasing force 79 of thepunch spring into contact with a rivet R supplied through a side port inthe nose 74 (see FIG. 8c). Further advance of the actuator shaft 80forces the rivet R into the sheet materials as shown in FIG. 8d. Duringthe final stages of full insertion of the rivet R the stop 82 on theplunger 72 comes into abutment with the shoulder 83 at the top of theguide bush 78 and any additional force applied by the actuator is evenlydistributed across the nose and the punch as a late clamping force. Theactuator shaft 80 is then retracted a short distance until the loadapplied by the punch to the rivet R is removed at which point there isstill a small clamping force applied by the nose by virtue of the actionof the compression spring 84 (FIG. 8e). When the punch 73 is unloadedthe actuator stops and the cam 85 is rotated by an external actuator ora biasing spring (not shown) to allow the punch 73 to retract slightly(under the influence of the punch spring 79) into the nose 74 so that itno longer projects therefrom (FIG. 8f).

The actuator is then operated a second time so as to advance the shaft80 again until the stop 82 on the plunger 72 abuts the shoulder 83 ofthe guide bush 78 (FIG. 8g). As a result of the orientation of the cam85 the punch 73 no longer extends out of the nose 74 and therefore doesnot contact the set rivet. Thus all the force applied by the actuator istransferred to the sheet materials via the clamping surface of the noseonly. The force applied by the actuator may be increased gradually untilthe desired clamping force is achieved. Finally, the actuator shaft isretracted to the start position and the cam is reset.

This method of applying a post rivet-insertion clamping force isadvantageous in that the force is imparted is not dependent on theC-frame stiffness. The required post rivet-insertion force is applied bysimply advancing the nose and driving deflection into the C-frame untilthe required clamping load is attained regardless of the distance oftravel of the lower limb of the C-frame. A load cell is associated withthe actuator in such a way as to measure the applied force is used tofeed control signals to a control system that governs the actuatoradvance. The control system can be self-calibrating.

In a modified approach the actuator shaft 80 may be held in position atthe full extent of the rivet insertion stroke and the cam 85 rotated byan external actuator whilst the punch 73 is still loaded with the fullrivet insertion force. This operation effectively transfers all theforce used to insert the rivet R into additional clamping force appliedonly by the nose without the need for a separate descent of the actuatorshaft.

In a modified design a disc spring (not shown) may be supported on theabutment shoulder 77 of the clamping cylinder 71. Towards the end of thestroke of the actuator shaft 80 its abutment shoulder 81 comes intocontact with the disc spring and further advance requires a force toovercome the bias of the disc spring. This force is transferred to theclamping cylinder 71 so that at the end of the rivet insertion thisspring will have the effect of providing an additional clamping forceduring the final stages of rivet insertion. This is desirable as theflow of sheet material out of the die cavity is restrained whilst therivet is still moving during the final stages of insertion and helps tobuild compressive stresses into the material around the rivet insertionlocation and this improve fatigue life of the resulting joint.

It will be appreciated that the retraction of the punch may be effectedby any appropriate device instead of the cam.

The same apparatus may be used to insert a rivet into sheet materialwithout any clamping force prior to rivet insertion but with a selectedclamping force during the final stages of rivet insertion. In such anoperation the rivet R is inserted into the end of the nose 74 such thatit projects therefrom (see FIG. 9a). The clamping cylinder 71 descendsas before and the rivet R is inserted without any downward movement ofthe plunger 72 and punch 73 (see FIGS. 9b to 9 d). When the rivet shankis fully inserted and the rivet head is just contacting the uppersurface of the sheet material 5 (see FIG. 9d) the nose clamping surface75 comes into contact with the sheet material 5 and imparts a clampingforce that is dependent of the characteristics of the disc spring (iffitted). The plunger 72 and punch 73 then travel as before to insert therivet R fully into the sheet material 5. The clamping force reaches itsmaximum when the disc spring (if fitted) is fully compressed between theabutment shoulders 81, 77 of the actuator shaft 80 and clamping cylinder71 and the plunger stop 82 abut the shoulder 83 of the guide bush 78.The post rivet-insertion clamping force is then applied, if required, asdescribed above.

In all embodiments the post rivet-insertion clamping force and theinsertion force are applied at separate stages in the riveting cycle andthus can be measured using a single transducer such as a load cell.Moreover, since the forces are never simultaneously applied to theC-frame the peak loading of the frame is never more than the highest ofthe two forces. This is in contrast to existing technology where theinsertion and principal clamping loads are applied simultaneously for atleast part of the riveting cycle.

Tests have established that for some joint types a significant reductionin the clamping force prior to rivet insertion can allow for a reducedrivet insertion force. Joints made in relatively thick sheets may bebest suited to low or zero clamping force prior to rivet insertion and arelatively high post rivet-insertion clamping force. In thinner joints alow to moderate clamping force prior to rivet insertion may benecessary. The present invention allows for the clamping force bothprior to, during and after rivet insertion to be selected and controlledaccording to the particular joint being formed. The clamping forceduring rivet insertion may be varied as described above by applying arelatively high clamping force as the head of the rivet moves flush withthe sheet material. If no clamping force is required prior to insertionthe nose need not be advanced with the punch before or during rivetinsertion. This means that the nose of the apparatus may be madeconsiderably shorter in length as it is only required to apply aclamping force after rivet insertion.

It is to be understood that the present invention has application toclinching technology such as that described in our aforementionedEuropean Patent No. 0614405.

It will be appreciated that numerous modifications to theabove-described designs may be made without departing from the scope ofthe invention as defined in the appended claims. For example, the maincylinder in the embodiment of FIG. 6 may alternatively have an internalscrew thread engaged by a drive mechanism to control advancementFurthermore, the embodiment described in relation to FIGS. 2 and 3 maybe modified such that the actuator 23 serves to hold the mechanicallinkage 20 with the aid of mechanical advantage, or the linkage 20 maybe moved to a position where it travels over-centre and locks thelinkage in place. The hydraulic locking device described earlier may beused in conjunction with a clamping cylinder that is moved under theinfluence of a spring force.

It is to be understood that the present invention has application to ahand-held riveting or clinching gun. Moreover, the method of inserting afastener of the present invention can be used not only in applicationsin which two or more sheets of material are to be joined but also hasapplication to the insertion of a fastener, such as a stud, into asingle sheet of material. Such a stud may be used as a fixing to connectto another component.

The invention may be used in conjunction with a self-piercing rivetinsertion actuator that operates to drive the rivet by multiple impactsat a pulsated excitation frequency such as that described in EuropeanPatent Application EP-A-0890397.

What is claimed is:
 1. A method for inserting a fastener (1) into sheetmaterial (5, 6) comprising inserting the fastener (1) into at least onesheet without full penetration such that a deformed end of the fastenerremains encapsulated by an upset annulus of the sheet material, thesheet material (5, 6) being disposed between a nose (17) and a die (16)of fastening apparatus (18) and the fastener (1) being inserted into thesheet material by means of a punch that is reciprocal relative to thenose (17), characterized in that a first clamping force is applied tothe sheet material during fastener insertion between the nose and die inthe region around the fastener insertion location, and in that after thefastener is inserted the punch is retracted and then a second clampingforce is applied to the sheet material (2, 3) between the nose (17) anddie (16) in the region around the fastener insertion location so as toreduce deformation of the sheet out of its plane in the region aroundthe fastener insertion location.
 2. A method for inserting a fasteneraccording to claim 1, the nose (17) being supported by a support member(20), characterized in that movement of the nose (17) relative to thesupport member (20) in a direction away from the die is prevented duringthe fastening operation.
 3. A method for inserting a fastener accordingto to claim 1, wherein a clamping force is also applied to the sheetmaterial (5, 6) prior to the fastener being inserted.
 4. A method forinserting a fastener according to claim 1, wherein the second clampingforce and the force required to insert the fastener (1) are derived fromthe same actuator.
 5. A method for inserting a fastener according toclaim 1, wherein the second clamping force applied after insertion ofthe fastener (1) is varied by varying the force applied to insert thefastener (1).
 6. A method for inserting a fastener according to claim 1,wherein the punch is retracted whilst the sheet material is under loadby the first clamping force.
 7. A panel clinching method wherein two ormore sheets of material (5, 6) are deformed into locking engagement, thesheet material being disposed between a nose (17) and a die (16) offastening apparatus, the sheet material (5, 6) being deformed by meansof a punch that is reciprocal relative to the nose (17), characterizedin that a first claming force is applied to the sheet material duringfastener insertion between the nose and die in the region around thefastener insertion location, and in that after the sheets are deformedby the punch, the punch is retracted and then a second clamping force isapplied to the sheet material between the nose (17) and die (16) in theregion around the punch insertion location so as to reduce deformationof the sheet (5, 6) out of its plane in the region around the punchinsertion location.